Water in Pinus Radiata Wood Secondary Cell Walls: An Investigation Using Nuclear Magnetic Resonance and Synchrotron X-Ray Diffraction

Abstract

The mechanical properties of wood allow it to be used for numerous purposes. For most purposes, drying of the wood material from the green state, sawn from the log, is first required. This drying step significantly improves the strength properties of wood. It is therefore clear that moisture in wood plays an important role in determining the bulk mechanical properties. Over the last century, many studies have been carried out to investigate the way in which the water content wood affects the bulk mechanical properties. More recent studies have focused to the individual chemical components that make up wood to understand the observed changes in bulk mechanical properties. Models of the nanostructure of wood contained; cellulose, hemicellulose, and lignin, and the arrangement and location of these components in terms of their mechanical properties was interpreted through what was described as the 'slip-stick' mechanism, by which wood, in its green state, maintained its molecular and mechanical properties under external stresses. This model, while insightful, failed to account for the presence and the role of water in the nanostructure of wood. In this work, synchrotron based X-ray diffraction and NMR studies, have been used to develop a new model, in which water plays a vital role in the determination of the mechanical properties of wood in its green, part-dried, and rewet states. X-ray diffraction showed that changes occur to the molecular packing of cellulose crystallites with change in moisture content, and that these changes begin to occur under mild drying conditions, i.e. drying in air at ambient temperatures. These changes depend on the severity of drying, whether ambient or forced oven drying, and are to some extent reversible. A spin-diffusion model was constructed using dimensions obtained from Xray diffraction, comparisons between predictions and experimental data from an NMR study showed that the location of water was dependent on the moisture history of wood. In the green state, at least some of the water in the wood cell wall forms a layer, between the cellulose crystals and the hemicellulose and lignin matrix. If dried and then rewet, this water associated with the cellulose crystals was not present to the same degree as in the green state, allowing a closer association of the hemicellulose with the cellulose. The effect of this change in water distribution in the wood cell wall on the bulk mechanical wood properties was shown in mechanical testing. The nanostructure of the wood cell wall therefore should be considered to contain cellulose, hemicellulose, lignin and water, where each component contributes, according to its molecular properties, dynamic mechanical properties which are reflected in the bulk material properties.